The second method of calculating climate sensitivity is more empirical. At the Vostok base in Antarctica, under a program of the National Ocean and Atmospheric Administration (NOAA), scientists have drilled into the ice to a depth of two miles. The ice samples they have retrieved show layers produced by seasonal differences in precipitation. As a result the scientists have a record of the Earth's climate for the past 420,000 years. Gas bubbles and isotopes within the ice layers reveal the relationship between the concentration of CO2 in the atmosphere and the temperature over that time.
In his film, Al Gore noted the close correlation between changes in CO2 and changes in temperature. In fact, the graphs fit almost perfectly. We need to consider the actual numbers, so I refer you to the graph below which is published by Davies & Co in their online article New Antarctic Ice Core Data:
This graph plots the results through four ice ages and five interglacial periods. The pattern is consistent with the increase in CO2 from 180 to 280 parts per million (ppm) associated with a rise in temperature at the poles of about 12°C 10°C, or 21.6°F 18°F. Notice that the increase in CO2 is less than double, yet the increase in temperature is greater than projected in the computer models above. This indicates that the estimates for climate sensitivity derived from computer models are likely to be conservative.
Ice core data also tells us the pre-industrial concentration of CO2, and NOAA has determined this to be 278 ppm. The current (late 2010) concentration of CO2 is 390 ppm, so the pre-industrial level was 112 ppm lower than today's levels. If you then factor in the contribution of the other greenhouse gases, most of which are man-made, our predicament becomes worse. The combined concentration of all greenhouse gases is currently back in 2006 was 433 ppm of CO2 equivalent, so we are well on the way to the doubling in CO2 discussed above.
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